Fluid drilling systems are known and use water under high pressure to cut solids such as soft rock, coal and the like. These water jet drilling systems are finding greater acceptance in the mining industry and can be used instead of the traditional mechanical cutting heads.
The known water jet drilling systems all have a cutting apparatus advanced by forces transmitted along a rigid drill string or in one instance by fluid pressure exerted on a piston type arrangement. The cutting apparatus has one or more water jet cutting nozzles on a leading portion of the apparatus.
Conventionally, in order to cut a circular hole, the rigid drill string is rotated thereby sweeping the forward cutting jets through a circular path.
More recently a more successful arrangement has been developed whereby the cutting jets alone are rotated by means of a swivel head powered by the thrust from the cutting jets, with all other parts of the drill stem leading to the cutting head being stationary. The apparatus is advanced by pushing on the rigid drill string with the rotating fluid cutting head cutting a hole in the solid.
More recently, a water jet drilling system has been developed which is effective in drilling in-seam boreholes of up to 300 m length or more with a rigid drill string. The major features of this drilling nozzle are:    a commercially available Woma FR47 high speed self-rotating water jet nozzle as the main cutting component, a stepless shroud cage to prevent the Woma FR47 nozzle from stalling (stop spinning),    a bent sub member to control the borehole trajectory,    a retro sub member to provide sufficient flow to flush the relatively large cuttings from the hole,    a nozzle cross over sub to connect the Woma FR47 nozzle to the retro sub member and the retro sub member to the drill string.
To advance the cutting apparatus and to provide the cutting fluid to the nozzle, drill rods of 3 m length, designed to withstand internal pressures of up to 1000 bar, were used. These drill rods were used as a conduit for the required supply of high pressure water to the drilling nozzle. (The rods were also used as a conduit for the required supply of high pressure water to the drilling nozzle). The rods were also used in association with a drilling rig to push or advance the nozzle into the borehole. To facilitate the removal of the cutting debris from the borehole, rearwardly facing flushing nozzles (or retro jets) were used. A high pressure water pump capable of a maximum pressure of 650 Bar at a flow rate of 160 liters per minute was used for this work.
In this more recent arrangement, the drilling technique involved the following steps:    align the drill rig to the desired borehole direction,    attach the Woma FR47 nozzle to the high pressure drill rods (the first 10 meters of borehole were drilled without the bent sub and retro assembly members. This is done to avoid unnecessary spray back from the flushing jets on the retro sub), after collaring 10 meters of borehole the drill string is withdrawn from the hole and the bent sub and retro assembly members attached behind the Woma FR47 nozzle, the nozzle assembly and drill string are re-inserted to the bottom of hole (BOH) and drilling continued, the nozzle assembly being advanced by pushing on the drill string with the rig.
A recognised advantage of fluid drilling systems is their propensity for Round the Corner or Ultra-Short Radius Drilling. These methods typically involve drilling horizontal holes radiating out from a vertical well. To allow Round the Corner Drilling, it is known to make the drill string tube up in steel segments each 45 cm long and hinged on the top surface. A drive chain is welded along the length of the string. As the segments come down the vertical well, they are disconnected on their lower side and are able to rotate around a drive cog at the bottom of the vertical well. Thus, the drill string would feed down the vertical well as a rigid unit and would also feed into the horizontal hole as a rigid unit.
The water cutting nozzle was powered by pressurised water which was fed through a high pressure hose. The high pressure hose either extended through the rigid drill string, or to one side of the drill string.
One method of Ultra-Short Radius Drilling involves a fluid drilling apparatus attached to a length of coiled tubing. The fluid drilling apparatus and coiled tubing are fed through a whipstock assembly which bends the tubing through an ultra-short radius bend (0.3 m radius). The tubing is thereby deflected laterally away from the vertical well by plastically deforming the tubing through a series of guides and rollers. The coiled tubing is used to supply the high pressure cutting fluid to the fluid drilling apparatus. The fluid drilling apparatus is forced into the formation to be drilled by means of a complicated piston arrangement which utilises the high pressure of the cutting fluid.
A difficulty with water cutting systems is ensuring that the nozzle assembly remains in the desired horizon as the apparatus is advanced by the rigid drill string. It is noted with conventional systems that there is a tendency for the cutting apparatus to drop relative to an horizon as it is advanced.
While not wishing to be bound by theory, it appears that the drop is caused by the drill string being rigid, or by the drill string otherwise being predominant in the advancement of the nozzle assembly.
To steer these devices, a bent sub is used and the rigid drill string is rotated to rotate the orientation of the bent sub and this provides a measure of steering to the system.
Drill strings formed from coiled tubing are known. The coiled tubing allows the drill string to exhibit some degree of flexibility. However the coiled tubing allows only a restricted amount of flex, and it is found that if the coiled tubing is forced around a whipstock, the tubing goes past its elastic limit which means that it is difficult to retrieve. The tubing must be cut-off electrochemically, or by some other means and therefore does not function as a flexible hose.
International Patent Application WO 95/09963 describes a drilling system. In this system a first drill string is pushed down a borehole and deflected horizontally via an elbow. The first drill string has a mechanical ball cutter and the drill string is rotated to rotate the ball cutter. This drill string is then removed, and a second flexible drill string is inserted down the bore hole and through the elbow.
The second drill string does not rotate and terminates with a relatively low pressure fluid cutter, cutting at about 3000-4000 psi. The fluid cutter slowly blasts a bore in the surrounding strata. There appears to be no method to advance the drill string into the horizontal bore other then by the weight of the drill string in the vertical or by conventional pushing on the drill string. The cutter has a more or less conventional low pressure retro jet assembly (about 3000-4000 psi) which functions to flush away the cuttings. The stated angle of the retro jets (45°) is consistent with a flushing action but at this angle the jets do not function to provide any meaningful forward thrust. In fact, it appears that the jets, if anything, may have an additional function to balance the kick back caused by the front non rotating nozzles, such that advancement is caused by the weight of the drill string.
As there is no advancement mechanism other then by the weight of the drill string, the fluid cutter advances very slowly with a stated cutting rate being 60 m in 6-10 hours even in soft rock.
As the drill string appears to effect the forward movement of the cutter, the problem of drop in the cutting angle may still occur, a problem found with rigid drill strings.
With no advancement means other then the weight of the drill string being apparent, there is a high probability that extended horizontal drilling will cause the flexible drill string to adopt what is known as “helical lock up” which is when the drill string can no longer be advanced by pushing on the drill string. This effect is probably why the horizontal hole lengths in the examples were limited to about 60 m.
In the examples, the drill string is a coiled steel tube of smaller than usual diameter (12.5 mm) to provide it with sufficient flexibility. With the small diameter tube, only small volumes of low pressure water can pass to the fluid cutter.
In draining methane from a coal seam, it is essential for the sake of efficiency to not change the permeability of the coal. Any reduction in the permeability will adversely effect methane drainage into the cut bore from the surrounding coal. It is known that surfactants reduce coal permeability and therefor, for drilling drainage holes in coal seams, the drilling system described above would not be suitable as surfactants are required.